Anaerobic aluminum-water electrochemical cell

ABSTRACT

Provided is a method for generating an electrical current. The method includes: introducing water between the anode and at least one cathode of an electrochemical cell, to form an electrolyte; anaerobically oxidizing aluminum or an aluminum alloy; and electrochemically reducing water at the at least one cathode. When the cell is in operation, the hydroxyaluminate (Al(OH)4−) in the electrolyte reaches a concentration maximum and thereafter a concentration minimum. The concentration maximum is above 125% of the saturation concentration and below 2000% of the saturation concentration. The concentration minimum is below 125% of the saturation concentration and above 50% of the saturation concentration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 14/096,810 filed Dec. 4, 2013 and claiming thebenefit of U.S. Provisional Patent Application No. 61/733,002 filed Dec.4, 2012. The disclosures of these applications are incorporated byreference herein in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.FA8721-05-C-0002 awarded by the U.S. Air Force. The U.S. government hascertain rights in the invention.

TECHNICAL FIELD

The present invention relates to electrochemical cells for use indynamic storage of energy, and more particularly, to anaerobicaluminum-water electrochemical cells.

BACKGROUND ART

Aluminum metal is an energy-dense (e.g., greater than 80 MJ/L) fuel withthe potential to enhance a variety of common systems. Because aluminumcan oxidize in water, it is especially promising as a power source forundersea devices, which are severely limited by the low energy densityof conventional anaerobic energy storage media (e.g., less than 1 MJ/Lfor Li-ion batteries). However, while recent advancements in thescalable de-passivation of aluminum have eliminated some barriers toeffective energy storage in aluminum, efficient energy conversion fromthe heat of reaction 2Al+6H₂O→3H₂+2Al(OH)₃+Q remains elusive. Thisdifficulty is mainly attributable to the slow kinetics of the reaction,which are not conducive to maintenance of the steep temperature gradientrequired for efficient thermal energy conversion. In electrochemicalAl-based devices, the continuous loss of some of the aluminum anode dueto parasitic corrosion reduces the energy density of the cell andshortens the self-discharge time of the system. Thus, previous attemptsto commercialize Al-air and Al-water fuel cells have failed, largely dueto the high anodic overpotentials and parasitic anodic corrosion thatreduces discharge efficiencies to ˜10-50 percent.

SUMMARY OF EMBODIMENTS

In accordance with a first set of embodiments of the invention, there isprovided a method for generating an electrical current. The methodincludes: introducing water between the anode and at least one cathodeof an electrochemical cell, to form an electrolyte; anaerobicallyoxidizing aluminum or an aluminum alloy; and electrochemically reducingwater at the at least one cathode. The electrochemical cell includes: aplurality of electrode stacks, each electrode stack featuring an anodeincluding the aluminum or aluminum alloy, and at least one cathodeconfigured to be electrically coupled to the anode; one or more physicalseparators between each electrode stack adjacent to the cathode; ahousing configured to hold the electrode stacks, the electrolyte, andthe physical separators; and a water injection port, in the housing,configured to introduce water into the housing. When the cell is inoperation, the hydroxyaluminate (Al(OH)₄ ⁻) in the electrolyte reaches aconcentration maximum and thereafter a concentration minimum. Theconcentration maximum is above 125% of the saturation concentration andbelow 2000% of the saturation concentration, and the concentrationminimum is below 125% of the saturation concentration and above 50% ofthe saturation concentration.

In accordance with a second set of embodiments of the invention, thereis provided an improved method for generating an electrical current. Themethod includes: introducing water between the anode and at least onecathode of an electrochemical cell, to form an electrolyte,anaerobically oxidizing aluminum or an aluminum alloy, andelectrochemically reducing water at the at least one cathode. Theimprovement includes: when the cell is in operation, thehydroxyaluminate (Al(OH)₄ ⁻) in the electrolyte reaches a concentrationmaximum above 125% of the saturation concentration and below 2000% ofthe saturation concentration and thereafter a concentration minimumbelow 125% of the saturation concentration and above 50% of thesaturation concentration.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be more readily understoodby reference to the following detailed description, taken with referenceto the accompanying drawings, in which:

FIG. 1 shows an anaerobic aluminum-water electrochemical cell accordingto embodiments of the present invention;

FIG. 2 schematically shows a series of anaerobic aluminum-waterelectrochemical cells in a multi-cell configuration according toembodiments of the present invention;

FIGS. 3A-3C schematically show one electrode stack in an aluminum-waterelectrochemical cell configuration according to embodiments of thepresent invention;

FIG. 4A schematically shows one electrode stack in an aluminum-waterelectrochemical cell configuration with a water miscible electrolyte,and FIG. 4B schematically shows a comparison of anion and waterconcentrations over the length of the stack in FIG. 4A according toembodiments of the present invention;

FIG. 5 schematically shows an aluminum-water electrochemical cellconfiguration with a liquid anode and a vertical stack of cathodesaccording to embodiments of the present invention;

FIG. 6 schematically shows an aluminum-water electrochemical cell systemaccording to embodiments of the present invention;

FIG. 7 schematically shows an aluminum-water electrochemical cell with asingle electrode stack according to embodiments of the presentinvention;

FIG. 8 schematically shows an aluminum-water electrochemical cell with asecond metal electrochemical cell according to embodiments of thepresent invention;

FIG. 9 shows an aluminum-water electrochemical cell configuration withan acidic electrolyte according to embodiments of the present invention;

FIG. 10 shows an aluminum-water electrochemical cell configuration withan alkaline electrolyte according to embodiments of the presentinvention;

FIGS. 11A and 11B show performance curves for the cell shown in FIG. 1according to embodiments of the present invention; and

FIGS. 12A and 12B show performance curves for the cell shown in FIG. 6according to embodiments of the present invention.

FIG. 13 shows performance curves for an aluminum-water cell with analkaline electrolyte at different KOH concentrations.

FIG. 14 is the performance curve for KOH at a concentration of 0.1 Mfrom FIG. 13, re-plotted at a higher resolution.

FIG. 15 shows current density-potential curves comparing the performanceof a pure aluminum anode to that of an anode containing certainimpurities.

FIG. 16 shows current density-time curves were potential curvescomparing the performance of a pure aluminum anode to that of an anodecontaining certain impurities.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Embodiments of the present invention provide an anaerobic aluminum-waterelectrochemical cell that harvests energy from the oxidation of aluminummetal in an aqueous electrolyte or a non-aqueous electrolyte having asmall amount of water. Aluminum reacts with the water at theanode-electrolyte interface, and hydrogen gas is produced at thecathode. As the cell is anaerobic, it does not contain molecular oxygen(O₂) in amounts that might significantly compete with the water forreaction with the anodic aluminum. The electrolyte may contain minoramounts of molecular oxygen dissolved therein, though preferably atconcentrations of less than 15 mg/L and above 0.001 mg/L. In someembodiments, the concentration of molecular oxygen in the electrolytecan fall to levels below 1 mg/L and above 0.01 mg/L, especially but notonly when the cell is in operation.

Using catalytic effects to separate the half-cell reactions, embodimentsof the present invention demonstrate energy densities of about 3.7-20MJ/L, up to 25 times the effective energy density of conventionalanaerobic energy storage media, e.g., Li-ion batteries, the currentstate of the art in underwater power sources, albeit with some powerlimitations. Details of illustrative embodiments are discussed below.

FIG. 1 shows an anaerobic aluminum-water electrochemical cell 10, andFIG. 2 schematically shows a series of anaerobic aluminum-waterelectrochemical cells 10 in a multi-cell configuration according toembodiments of the present invention. As used herein, the termelectrochemical cell may encompass an individual electrochemical cell orcell unit, such as shown in FIG. 1, as well as configurations having anarray of electrochemical cells, such as shown in FIG. 2. Referring toFIGS. 1 and 2, the electrochemical cell 10 includes a number ofinterdigitated electrode stacks 12 of thin anodes 14 and cathodes 16separated by one or more physical separators 18. The anode 14 includesan aluminum or an aluminum alloy and may be in a solid phase (as shownin FIGS. 1 and 2), e.g., formed into a plate, or a liquid phase (asshown in FIG. 5 and discussed in more detail below). The electrode stack12 may include two cathodes plates 16 on either side of an anode plate14, such as shown in FIG. 2, although any number of cathode plates maybe used within each stack. In addition, any number of stacks 12 may beused within the electrochemical cell 10 depending on the application anddesired size. The stacks 12 can be parallel to one another (as shown inFIGS. 1 and 2) perpendicular to one another (not shown), or acombination of both configurations. The anodes 14 and cathodes 16 areelectrically connected by leads 34 through which electrons may pass toan external source or sink (not shown).

The electrochemical cell 10 also includes an electrolyte 20, e.g., abasic or acidic aqueous media or non-aqueous, water miscible media,disposed between the anodes 14 and cathodes 16, and a housing 24configured to hold the electrode stacks 12, the physical separators 18,and the electrolyte 20. In some embodiments, the electrolyte 20 may beprovided already inside the cell. In another set of embodiments, thecell 10 is kept dry until such time as established by the user, therebyminimizing corrosion reactions that might occur if the cell 10 wasstored with the electrolyte 20 in physical contact with the electrodes.Then, when a source of electric power is needed, the electrolyte 20 isintroduced between the anode 14 and cathodes 16, for example by floodingthe cell with pre-mixed electrolyte or by introducing the ingredients ofthe electrolyte 20 in the cell, and an electrical circuit is establishedbetween the leads 34. In some instances, the electrochemical cell 10 isprovided with the ingredients of the electrolyte 20 minus the solvent,which is added when the electrochemical cell 10 is activated. In oneexemplary method, water either pure or in the form of an aqueoussolution such as seawater is introduced into the housing 24 throughinjection port 22, forming electrolyte 20 by mixing with the otheringredients either prior to or after entering the housing 24.

The housing 24 may be made from any electrically insulating,non-reactive material, such as a plastic material (e.g., high-densitypolyethylene (HDPE), low-density polyethylene (LDPE), polypropylene(PP), or acrylonitrile butadiene styrene (ABS)), that iscorrosion-resistant to the electrolyte 20 and the two electrode 14 and16 materials. In particular, it has been found that the housing 24 canbe easily manufactured by ultrasonically welding panels of ABS. Thehousing 24 includes one or more water injection ports 22 configured tointroduce water, either pure or as part of an aqueous solution, into thehousing 24. Preferably, the water is injected into the electrolyte 20close to the cathode 16 and away from the anode 14. For example, thewater injection ports 22 may be configured to introduce the water intothe housing 24 so that the water flows through the physical separators18 next to the cathodes 16. This configuration increases theconcentration of water near the cathode 16, increasing the rate at whichwater is split into H⁺ and OFF, and decreases the concentration of waterat the anode 14, minimizing the parasitic corrosion reaction. Inembodiments of the present invention, the electrochemical cell 10undergoes two primary reactions, as shown in FIGS. 3A through 3C:3H₂O+3e ⁻→1.5H₂+3OH⁻ (cathode)  1)Al+3OH⁻→Al(OH)₃+3e ⁻ (anode)  2)yielding a total reaction of: Al+3H₂O→1.5H₂+Al(OH)₃

Referring again to FIG. 2, the water injection ports 22 are in fluidcommunication with a water supply 30. The water supply 30 may be aninternal tank that stores water or may be an external supply drawn fromoutside the electrochemical cell 10, such as seawater. The housing 24also includes one or more hydrogen release valves 28 configured to allowthe hydrogen generated at the cathode 16 to be released from theelectrochemical cell 10. Aluminum hydroxide waste 26 forms on thealuminum anode 14 when the aluminum reacts with the hydroxide ions. Thealuminum hydroxide waste 26 can reduce the performance of the fuel cell10. To reduce this effect, the hydrogen flow from the cathodes16 may bechanneled over the anodes 14, carrying the waste away from the anodesurface and into the electrolyte 20. The waste 26 may be removed fromthe electrolyte 20 using filters and similar technologies or may beallowed to build up in the electrolyte 20. If waste removal is desired,the individual cells 10 may each have a separate waste removal system ormay have a shared waste removal system 32. For example, the waste 26 maybe carried over and around the cells by the flow of hydrogen beforesettling to the bottom of the housing 24, where it may be removed orbecome trapped in a low-conductivity water immiscible fluid, such asmineral oil. The waste 26 or the fluid with the waste may be pumped outto a shared waste filter/ejection system 32 (such as described in moredetail below in FIG. 6). If the waste 26 is allowed to build up in theelectrolyte 20 or some electrolyte 20 is lost during operation, theelectrolyte 26, or some portion thereof, may be replaced periodically.

As mentioned above, the electrolyte 20 may include basic or acidicaqueous media or non-aqueous, water miscible media. For example,seawater or water, alkaline KOH or NaOH solution, acidic HCl or H₂SO₄solution, or alkaline ionic liquid or primary alcohol (e.g., methanol orethanol) solutions, and mixtures thereof, may be used as the electrolyte20. The goal of the electrolyte 20 is to allow the transport ofhydroxide ions without allowing water to react directly with thealuminum fuel in the anode 14. Thus, further additions may be made tothe electrolyte 20 to increase the power output and reduce the corrosionrate of the cell 10. For example, a surfactant, e.g., Triton X, orsodium dodecyl sulfate, may be added to increase the rate at whichhydrogen bubbles are shed from the surface of the electrodes 14, 16,ensuring that most or all of the surface area remains available toreactants and reduces bubble-overpotential caused by the drop in ionconcentration in a region filled with bubbles. In addition, thesurfactant reduces the size of the bubbles that are formed, which allowsthe electrodes 14, 16 to be kept closer to one another. To reducecorrosion, inert compounds, such as ionic liquids (e.g.,1-ethyl-3-methylimidazlium hydroxide and 1-butyl-3-methyl imidazoliumtetraflouroborate) may be added to the electrolyte 20 to decrease thewater activity of water molecules in the solution. This reduction inmobility helps trap the water molecules near the cathode 16, furtherreducing the water concentration near the anode 14 which causesparasitic corrosion. Other liquids, such as primary alcohols (e.g., 60%methanol), secondary alcohols (e.g., 2-propanol), acetonitrile (e.g.,30% ACN), dimethyl carbonate, and dimethyl sulfoxide may also be addedto the electrolyte in order to promote good OH− ion (anion)conductivity. The anion conductivity can be increased by dissolving abase, such as KOH, into the electrolyte 20. The ionic liquids,surfactant, and other liquids may be added in varying amounts to theelectrolyte 20, e.g., from about 5-95 vol % of the total electrolyte.

In some embodiments, the electrolyte 20 is a solution of aqueous orwater-miscible media containing a hydroxide base. It has been foundthat, in certain instances, the presence of a hydroxide base improvesthe performance of the cell 10. Without being bound to any particulartheory, it is believed that increasing the concentration of the OH−(hydroxide) ion in the electrolyte 20 facilitates the transport ofhydroxide ions from the cathode to the anode 14 and the removal of spentreactants while preventing or reducing reaction of water directly withthe aluminum fuel in the anode 14, thereby increasing the power outputand reducing the corrosion rate of the cell 10. Example hydroxide basesare provided by alkali metal bases, such as NaOH and KOH, alkaline earthmetal bases, such as Ca(OH)₂ and Mg(OH)₂, and combinations thereof.Hydroxide bases may also be generated in situ by mixing an aqueoussolution with one or more strong bases, for example MeONa, EtONa,n-BuLi, LDA, LDEA, NaNH₂, NaH, ((CH₃)₃Si)₂NLi. The strong base reactscompletely with water, yielding hydroxide anion (OH⁻) and substantiallynone of the original anion remains after the strong base is added to thesolution. Unless otherwise noted, the term “a hydroxide base” isintended to cover one or a combination of two or more hydroxide bases,depending on the context.

In a number of representative embodiments, the cell 10 includes anamount of hydroxide base sufficient to form an electrolyte 20 having ahydroxide base concentration of at least 0.5% to at most 13% of thesaturation concentration when a solvent, for example water, isintroduced between the anode 14 and the least one cathode 16, where thesaturation concentration is herein intended as measured at roomtemperature. Preferably, the amount of hydroxide base is sufficient toform an electrolyte 20 having a hydroxide base concentration of at least1% to at most 12% of the saturation concentration. More preferably, theamount of hydroxide base is sufficient to form an electrolyte 20 havinga hydroxide base concentration of at least 1.5% to at most 10% of thesaturation concentration, and yet more preferably the amount ofhydroxide base is sufficient to form an electrolyte 20 having ahydroxide base concentration of at least 7% to at most 13% of thesaturation concentration. The hydroxide base may be already pre-mixedwith the other ingredients of the electrolyte 20 or instead combinedwith such ingredients at the time the cell 10 is activated.

In another set of representative embodiments, the electrolyte 20contains a hydroxide base at a concentration of at least 0.05 M and atmost 3 M. In a third set of representative embodiments, the hydroxidebase concentration is from at least 0.1 M to at most 2.5 M. In a furtherset of representative embodiments, the hydroxide base concentration isfrom at least 0.25 M to at most 2 M. In additional embodiments, theelectrolyte 20 includes a hydroxide base at a concentration from atleast 0.5 M to at most 1.5 M, and, in further embodiments, the hydroxidebase concentration is from at least 0.5 M to at most 1 M. Theingredients of the electrolyte 20 may be pre-mixed or instead combinedat the time the cell 10 is activated.

When the cell 10 is in operation, that is, generating an electriccurrent, it is preferred that there be substantially no homogeneousprecipitation of aluminum hydroxide waste 26 because it tends to formprecipitate everywhere in the electrolyte 20 and therefore fouls theanode and cathode surfaces, clogs tubes, and jams the pumps ofelectrolyte circulation systems. It has been found that homogeneousprecipitation can be minimized if the concentration of aluminum speciesin the electrolyte 20 is maintained between at least 0.01 M to at most0.7 M, where the term “aluminum species in the electrolyte” refers toaluminum-bearing species dissolved in the electrolyte 20, including butnot limited to aluminum hydroxide (Al(OH)₃), hydroxyaluminate anion(Al(OH)₄ ⁻), and oxyanions of aluminum such as AlO₂ ⁻. Preferably, theconcentration of aluminum species in the electrolyte 20 is maintainedbetween at least 0.1 M to at most 0.6 M. More preferably, theconcentration of aluminum species in the electrolyte 20 is maintainedbetween at least 0.2 M to at most 0.5 M.

The volume of the electrolyte 20 in the electrochemical cell 10 shouldbe kept roughly constant to ensure that the electrodes 14, 16 remaincovered. If the electrolyte 20 is volatile, hydrogen may be allowed tobuild up at the top of the chamber 24, increasing the pressure andreducing electrolyte evaporation. If the electrolyte 20 is lost to sidereactions or as part of the process of removing waste 26, asupplementary tank of electrolyte 20 may be used to ensure that thevolume is maintained and the housing 24 remains filled. The watercontent of the electrolyte 20 directly controls the power output of thefuel cell 10. Therefore, the power may be increased or decreaseddepending on the water injection rate.

FIG. 4A schematically shows an electrochemical cell 10 configurationwith a water miscible electrolyte 20. In this embodiment, theelectrolyte 20 is a non-aqueous, water miscible material with good OH−ion (anion) conductivity. For example, ionic liquids and light alcohols,such as methanol, may be used. As shown, water may be gradually injectedinto the electrolyte 20, preferably near the cathode 16. Thenon-aqueous, water miscible electrolyte may have about 90 wt % of wateror less. In addition, the water or the electrolyte 20 may have one ormore of the additives mentioned above, e.g., ions dissolved in it, topromote ion conductivity, adjust power output, and reduce the corrosionrate of the fuel cell. Due to the low water content of the electrolyte20, parasitic corrosion of the aluminum or aluminum alloy anode 14 issignificantly reduced since the concentration of water at the anode 14is reduced (as shown in FIG. 4B), making the half-cell reactions muchmore favorable over the parasitic reaction.

As mentioned above, the anode 14 material includes an aluminum oraluminum alloy that may be in a solid phase or a liquid phase. When inthe solid phase, the aluminum or aluminum alloy anode 14 may be in theform of a thin plate, as shown in FIGS. 1 and 2. The thickness of theanode 14 may vary depending on the power or energy density requirements,e.g., for higher power density, a thinner anode may be used and forhigher energy density the mass of the anode may be increased.Preferably, the anode thickness may be about 1-3 mm. Any purity ofaluminum may be used, but it has been found that aluminum having apurity of at least 99.95 wt % can improve the coulombic efficiency ofthe cell. Thus, high purity aluminums, for example “4N aluminum” havinga purity of 99.99 wt %, and “5N aluminum” having a purity of 99.999 wt %are particularly advantageous in a number of embodiments.

In some embodiments, the anode 14 is substantially free of certainimpurities, such as iron and copper, which may decrease the energydensity of the fuel cell by increasing the rate of parasitic corrosion.In certain embodiments, the anode 14 is substantially free of titaniumand boron as the presence of those elements in an anodic aluminum alloyhas been found to provide no benefit and detrimental in some cases. Asused in this description and the accompanying claims, the term“substantially free” of a given component shall have the meaning of thecomponent being present in an amount less than 1 ppm, unless the contextotherwise requires.

The goal of the anode 14 is to avoid passivation with both Al₂O₃ andAl(OH)₃ as well as hinder the H₂ evolution reaction. Alloying thealuminum with a metal with a high hydrogen overpotential and a highernobility than aluminum in the electrochemical series (e.g., indium)reduces the corrosion of the aluminum metal and may increase thedischarge potential. Alloying the aluminum with a metal that disruptsthe alumina passivation layer which covers the anode 14 (e.g., gallium)increases the current density. Combinations of metals can be alloyedwith aluminum to achieve a mixture of effects, e.g., Al or Al alloy withGa, In, Sn, and/or Mg. Preferably, the anode 14 is made of an aluminumalloy with about 0.1 wt % of In and 0.1 wt % of Ga. In embodiments usinga solid anode, the electrochemical cell 10 is mechanically recharged byreplacing the solid aluminum or aluminum alloy anodes 14.

When in the liquid phase, the anode 14 material may be comprised of aliquid metal alloy that includes aluminum. The liquid metal (e.g., analloy comprised of Ga, In, Sn, and/or Mg) is not consumed in theanode-side reaction. Rather, the liquid metal merely facilitates thepassage of aluminum fuel to the anode-electrolyte interface. Forexample, the liquid material may be about 100 wt % gallium or may beabout 65-70% wt % Ga, 20-25% wt % In, and 5-15 wt % Sn. Advantages ofthis embodiment over solid-anode technology are that it provides ahigher standard cell potential vs. both oxygen reduction and hydrogenevolution electrodes and a significantly slower rate of anodic corrosionrelative to the rate of galvanic discharge. The oxidation of aluminum inthe electrochemical cell 10 can proceed through either theelectrochemical pathway described above and shown in FIGS. 3A through3C, or through the corrosion reaction 2Al+6H₂O→3H₂+2Al(OH)₃ occurringentirely on the anode 14. In addition to the depassivation effectmentioned above, the liquid anode 14 may be preferable to a solidaluminum or aluminum-alloy anode 14 because the liquid anode 14configuration retards this corrosion reaction. This effect may beattributable to the low surface activity of liquid metal surfaces, inparticular the alloys of Ga, In, and Sn, for the hydrogen evolutionreaction in both basic and acidic aqueous and non-aqueous media. Inaddition, the liquid phase anode 14 may increase the open-circuitpotential of the anode 14 relative to the cathode 16 or hydrogenelectrode. Whereas solid Al alloys typically passivate with a thickAl(OH)₃ gel or layer during anodic polarization, convection on theliquid metal anode surface reduces this effect, and open circuitpotentials up to −1.55V vs. H₂—H₂O may be observed.

As shown in FIG. 5, a solid aluminum or an aluminum alloy material 36may be fed into the liquid material via an in-situ interdiffusionprocess, such as described in U.S. Patent Application Publication No.2013/0276769, incorporated by reference herein in its entirety.Preferably, the rate of reaction at the liquid anode-water interface(point 1 in FIG. 5) is equal to the rate of diffusion of solid fuelmaterial 36 into the liquid anode 14 at the fuel feed (point 2 in FIG.5). A high surface-area cathode material 16, such as Pt-loaded carbonpaper, felt, cloth, or mesh (as will be described in more detail below),is separated from the liquid anode material 14 by the surface propertiesof the electrolyte 20 and liquid metal anode 14. In this case,additional physical separators 18 do not need to be included, althoughthey may be used. The stack of cathodes 16 may be arranged in a verticaldirection so that one end of each cathode is surrounded by the liquidanode 14. The fuel supply material 36 may be any arbitrary size relativeto the reactor vessel 24 and may consist of pure aluminum or an aluminumalloy with In, Mg, Sn, and/or Ga. For example, the aluminum alloy may bealuminum with about 0.1 wt % of In and 0.1 wt % of Ga. The solidaluminum or aluminum alloy material 36 may be introduced into the liquidanode material as a wire, foil, block, pellets, or a combinationthereof. In addition, an additive may be added to the liquid material inorder to facilitate the dissolution of the aluminum or the aluminumalloy in the liquid. The electrical lead 34 to the anode (point 5 inFIG. 5) may be made with any electrically conductive material, such as asmall diameter wire or razor blade, preferably composed of a materialsuch as tungsten which easily wets to the liquid metal alloys in theanode 14.

Whether using a solid or liquid anode 14, the cathode 16 may be made ofany material with a low hydrogen overpotential which is chemicallystable in the chosen electrolyte 20. For example, nickel and platinumboth have low hydrogen overpotentials, although platinum is not stablein methanol. In addition, the cathode 16 preferably has a high surfacearea to decrease the current density on its surface and reduceoverpotential losses. This may be achieved by selecting materials withhighly engineered surfaces, such as carbon paper, felt, cloth, or meshmaterial, and then depositing Ni or Pt on its surface. For example, thecathode 16 may be Pt coated carbon or titanium or a NiC matrix material.The cathode 16 may be in the form of a thin plate that is spaced apartfrom the anode 14. The thickness of the cathode 16 may vary depending onthe power or energy density requirements, and one or more cathodes 16may be used in the electrode stack. Preferably, the cathode thicknessmay be about 40-100 μm. Additional anion permeable polymer layers, suchas nafion or anion exchange membranes, may be added to the cathode 16 ifions in the electrolyte 20 tend to deposit on and contaminate thecathode 16.

It has also been found that the electrochemical roughness factor of thecathode 16 can influence the performance of the cell 10. Theelectrochemical roughness factor of a material is determined bymeasuring its effective double layer capacitance and comparing it tothat of an electropolished flat foil of the same material. (SeeWaszczuk, P., Zelenay, P. & Sobkowski, J. Surface interaction ofbenzoic-acid with a copper electrode. Electrochim. Acta 40, 1717-1721(1995)). Preferably, at least one cathode 16 has a surface characterizedby an electrochemical roughness factor of at least 5. More preferably,the electrochemical roughness factor of the cathode surface is at least10, and most preferably at least 15. While there is no preferred upperlimit to the electrochemical roughness factor of the cathode, anacceptable upper boundary may be set at 25, 40, or 50.

The mean pore diameter of the cathode surface has also been discoveredto influence the performance of the cell 10. Preferably, at least onecathode 16 has a surface characterized by a mean pore diameter of atmost 50 μm. More preferably, the mean pore diameter of the surface is atmost 100 μm. While there is no preferred lower limit to the mean porediameter, an acceptable lower boundary may be set at 5 μm, 10 μm, or 25μm.

In a further discovery, it has been found that higher voltages can beachieved when the concentration of hydroxyaluminate (Al(OH)₄ ⁻) in theelectrolyte 20 first grows to reach a maximum, then drops to a minimum.Preferred maxima and minima vary with factors associated with theoperating conditions of the cell such as the composition of theelectrolyte 20 and the power draw rate. Usually, a higher electrolyte pHis accompanied by a higher preferred hydroxyaluminate concentrationmaximum, and vice versa, while higher power output rates are associatedwith higher minima. In representative embodiments, and depending on theoperating conditions, the maximum may fall within a range from at least125% of the saturation concentration to at most one of 150%, 175%, 200%,225%, 250%, 500%, 750%, 1000%, 1500%, or even 2000% of the saturationconcentration. The minimum may vary between at most 250% of thesaturation concentration to at most one of 225%, 200%, 175%, 150%, 125%,100%, 75%, 50% or even 30% of the saturation concentration.

In some instances, better performance is obtained when thehydroxyaluminate concentration of the electrolyte 20 is kept withincertain ranges when the cell is in operation. In an exemplary set ofembodiments, the hydroxyaluminate concentration of the electrolyte 20 ismaintained between at least 20% to at most 750% of the saturationconcentration. Preferably, the electrolyte hydroxyaluminateconcentration is maintained between at least 30% to at most 500% of thesaturation concentration. More preferably, the electrolytehydroxyaluminate concentration is maintained between at least 50% to atmost 250% of the saturation concentration. Most preferably, theelectrolyte hydoxyaluminate concentration is maintained between at least50% to at most 150% of the saturation concentration.

The one or more physical separators 18 may be made of any material witha relatively high electrical resistivity which is chemically stable inthe chosen electrolyte 20, such as a plastic material (e.g., HDPE orLDPE). For example, the electrical resistivity may be greater than about10⁸ ohms·cm. In addition, the physical separator 18 preferably has ahigh areal density (e.g., mostly open area), so that the water may beintroduced and allowed to flow through the physical separator 18. Forexample, the physical separator 18 may be made with a mesh materialhaving about 95% or greater areal density (e.g., thin strands of apolymer material), preferably having openings of about 100 μm or larger.In addition, the physical separator 18 may be in the form of a thinplate disposed adjacent to the cathode 16. The thickness of the physicalseparator 18 may vary, but is preferably about 200 μm or less.

FIG. 6 schematically shows an aluminum-water electrochemical system 40that uses the electrochemical cell 10 according to embodiments of thepresent invention. The system 40 includes the electrochemical cell 10, awaste separation system 32 in fluid communication with theelectrochemical cell 10, and a fuel injector 38 in fluid communicationwith the waste separation system 32 and the water injection port(s) 22.The waste separation system 32 is configured to receive the electrolyte20 and the aluminum hydroxide waste 26 from the electrochemical cell 10,e.g., at periodic times or when the electrolyte 20 is determined to havesufficient waste build up. The waste separation system 32 then separatesthe waste 26 from the electrolyte 20, and provides the cleaned upelectrolyte 20 to the fuel injector 28. The fuel injector 38 receivesthe electrolyte 20 from the waste separation system 32 and receiveswater from a water supply 30, e.g., an external supply such as seawater.If the electrochemical cell 10 includes a water supply 30 that isinternal to the cell 10, then the fuel injector 38 provides theelectrolyte 20 to the cell 10, which is then combined with the waterfrom the internal water supply 30.

Although the electrochemical cell 10 discussed above includes a numberof electrode stacks 12 with anodes 14 and cathodes 16, a single stack 12may also be used, such as shown in FIG. 7. In addition, a second metalelectrochemical cell 42, such as a Li—H₂O cell 42, may be used inconjunction with the aluminum-water electrochemical cell 10 discussedabove, such as shown in FIG. 8.

EXAMPLES

To further illustrate embodiments of the present invention, thefollowing non-limiting Examples are provided.

Example 1 Electrochemical Al—H₂O Reactor in Acidic Electrolyte

An acidic cell was created in 30 mM HCl electrolyte with a Pt coatedcathode, as shown in FIG. 9. Voltages ranging from 0.8 to 1.5 V wereproduced between the electrodes, with 95% of H₂ production occurring onthe Pt cathode, rather than the Al—Ga anode surface. The cell wasoperated at room temperature. While the exact half-cell reactions inthis system are not known exactly, the following hypothetical pathway isconsistent with observations to date:

Rxn 1 (Al + Ga anode): 2Al + 6H₂O→ 2Al(OH)₃ + E = −1.49 V 6H⁺ + 6e⁻ Rxn1.5 (HCl solution): Al(OH)₃ + 3H⁺ → Al³⁺ + E = 0 3H₂O Rxn 2 (Ptcathode): 3H⁺ + 3e⁻ → 1.5H₂ E = 0

Example 2 Electrochemical Al—H₂O Reactor in Alkaline Electrolyte

An electrochemical cell was constructed using an Al—Ga anode and Ptcoated cathode with a 0.5M NaOH electrolyte, as shown in FIG. 10. Thiscell produced 1.6V with approximately 90% H₂ production on the Ptcathode. The cell was also observed to produce 300 mW through a 5-Ohmresistor for an efficiency of ˜65%. The cell was operated at roomtemperature. The following pathway is confirmed by the XRD data of anelectrolyte precipitate and is consistent with the observed open circuitvoltage, the absence of solid hydroxide precipitate, and the steadydecrease in electrolyte pH:

Rxn 1 (Al + Ga anode): 2Al + 8OH⁻ → 2Al(OH)₄ ⁻ + 6e⁻ E = −2.33 V Rxn 2(Pt cathode): 6H₂O + 6e⁻ → 3H₂ + 6OH⁻ E = −.82 V

Example 3 Various Electrolyte Compositions

Electrochemical cells were constructed using an Al—Ga anode and Ptcathode with various electrolyte mixtures. The following performancemetrics were measured for the electrolyte mixtures:

TABLE 1 Electrolyte I_(Corr)/I_(SC) V_(OC) (V) I_(SC) (mA/cm²) 5M HCl(H₂O) .7 1.21-1.6  140 5M NaOH (H₂O) .6 1.21-1.55 94 .5M KOH (EtOH) .021.51 .4 5M KOH (MeOH) .01  1.1-1.55 12 KOH (BMIM-Tf₂N) 0 1.72 .002 AlCl₃(BMIM-PF₆) 0 0.9 .4In Table 1, I_(SC) is the short-circuit current, V_(OC) is theopen-circuit voltage, and I_(corr) is the corrosion current.

Example 4 Small Reactor Cell (44 mm³)

An electrochemical cell was constructed usingAl_(99.7)Ga_(0.15)In_(0.15) anodes and platinized titanium cathodes withan 0.5M KOH in H₂O₄₀Methanol₂₀Acetonitrile₄₀ electrolyte, as shown inFIG. 1. This cell produced about 1.9 W/L power density, 3.7 MJ/L (@1.2W/L) energy density, and had a neutral buoyancy of about 1.2 g/cm³.FIGS. 9A and 9B show the performance curves for the cell that was runfor about 30 hours.

Example 5 Large Reactor Cell (330 ml Cylinder)

An electrochemical cell was constructed using Al anodes and Ni (Nielectroplated on carbon) cathodes with a 1M KOH (aq) electrolyte, asshown in FIG. 6. This cell produced about 6 W/L power density and 5 MJ/L(@1.2 W/L) energy density. FIGS. 10A and 10B show the performance curvesfor the cell that was run for about 30 hours.

Example 6 Alkaline Aqueous Electrolytes

An electrochemical cell was constructed withAl_(0.998)Sn_(0.001)Mg_(0.001) anodes, Ni (Ni electroplated on carbon)cathodes, and an alkaline aqueous electrolyte at a number of KOHconcentrations. For each KOH concentration, the power density of thecell was measured at different voltages and the measurements wereplotted in FIG. 13, revealing an increase in performance directlyproportional to the electrolyte KOH concentration. This observation isconsistent with the cell generating current by the electrochemicalpathway described above in Example 2, where higher hydroxide baseconcentrations facilitate the transport of hydroxide ions and removal ofspent reactants while preventing or reducing reaction of water directlywith the aluminum fuel in the anode, thereby increasing the power outputand reducing the corrosion rate of the fuel cell.

As illustrated FIG. 14, which is a plot of the measurements obtainedwith KOH 0.1 M at a higher resolution than in FIG. 13, a beneficialeffect was already measurable at relatively lower alkali concentrations,while the results reported in Example 3 above show that such effects areobtainable with hydroxide bases at concentrations of 5 M and higher.

Example 7 Iron Impurities

A number of electrochemical cells were constructed using Al anodes andNi (Ni electroplated on carbon) cathodes with a 1M KOH (aq) electrolyte.Each electrode had a surface of 1 cm² and the electrolyte had a volumeof 5 mL. Different anode formulations were tested by alloying the anodicAl with different amounts of Fe, Si, Mg, and Sn. The coulombicefficiency of each cell was measured at the temperatures of 10° C. and30° C., respectively. The composition of each anode and the measuredcoulombic efficiencies are reported in Table 2:

TABLE 2 Fe Si Mg Sn 10° C. 30° C. Alloy wt % wt % wt % wt % Al mA/cm²Coul eff. mA/cm² Coul eff. 1 0.0005 0.0022 0.11 0.073 5N 16 0.98 42 0.992 0.0008 0.0014 0.105 0.073 4N6 7 0.98 22 0.84 3 0.0042 0.0044 0.1080.073 4N 7 0.98 20 0.82 4 0.0218 0.0219 0.11 0.073 3N6 7 0.97 22 0.82In Table 2, the amount of each impurity is measured in wt %, and thecoulombic efficiency is measured following discharge at the currentdensities reported in the table. It can be seen that the coulombicefficiencies depend on the purity of the Al used and that alloys wherethe aluminum is at least 99.95 wt % pure tend to yield better results.It is also noteworthy that increases in the Fe content of the anode leadto marked losses in coulombic efficiency, especially at 30° C. Suchlosses were less apparent at 10° C., likely because corrosion reactionsproceed at lower rates when the temperature is lowered.

Example 8 Titanium and Boron Impurities

A first electrochemical cell was constructed using an Al alloy (Alloy1066) anode and a Ni (Ni electroplated on carbon) cathode with a 1M KOH(aq) electrolyte. Each electrode had a surface of 1 cm² and theelectrolyte had a volume of 5 mL. The anodic alloy cell included 99.83wt % Al (from Al 5N, 99.999 wt % purity), 0.1 wt % Mg, and 0.07 wt % Sn.A second cell was constructed with an anode featuring the same alloy asthe first cell but with TiB₂ added in the amount of 0.01 wt %. The cellswere tested with a Biologic SP-50 potentiostat (Bio-Logic, France) usinga linear voltage sweep technique. Both electrode voltages were monitoredvs. a Hg/HgO reference electrode.

Current density-potential curves were calculated and plotted in thegraph shown in FIG. 15. Current density-time curves were also calculatedand plotted in the graph shown in FIG. 16. As seen in FIGS. 15 and 16,there is no benefit to, and some detriment from, the addition of Ti andB to the aluminum alloy of the anode.

Although the above discussion discloses various exemplary embodiments ofthe invention, it should be apparent that those skilled in the art canmake various modifications that will achieve some of the advantages ofthese embodiments without departing from the true scope of theinvention.

The invention claimed is:
 1. A method for generating an electricalcurrent using an electrochemical cell, the method comprising: providingthe electrochemical cell, the electrochemical cell comprising: aplurality of electrode stacks, each electrode stack comprising a solidaluminum or aluminum alloy anode, in the form of a plate, the platehaving a first side and a second side opposite to the first side, and atleast one cathode disposed directly next to the first side of the plateand at least one cathode disposed directly next to the second side ofthe plate, wherein the cathode on the first side and the cathode on thesecond side are configured to be electrically coupled to the anode; oneor more physical separators, between each one of the electrode stacks,positioned between the cathode on the first side of one electrode stackand the cathode on the second side of an adjacent electrode stack; ahousing configured to hold the electrode stacks, an electrolyte, and thephysical separators; and a water injection port, in the housing,configured to introduce water into the housing, so that the water flowsthrough the physical separators; introducing water between the anode andat least one cathode on the first side and between the anode and the atleast one cathode on the second side in each electrode stack, ahydroxide base and the water forming the electrolyte; anaerobicallyoxidizing the aluminum or aluminum alloy anode; electrochemicallyreducing the water at the at least one cathode; operating theelectrochemical cell such that hydroxyaluminate (Al(OH)₄ ⁻) in theelectrolyte reaches a concentration maximum and thereafter aconcentration minimum, wherein: the concentration maximum is above 125%of a saturation concentration and below 2000% of the saturationconcentration, and the concentration minimum is below 125% of thesaturation concentration and above 50% of the saturation concentration.2. The method according to claim 1, wherein the concentration maximum isabove 150% of the saturation concentration and below 1500% of thesaturation concentration.
 3. The method according to claim 1, where theconcentration maximum is above 175% of the saturation concentration andbelow 1000% of the saturation concentration.
 4. The method according toclaim 1, where the concentration maximum is above 200% of the saturationconcentration and below 500% of the saturation concentration.
 5. Themethod according to claim 1, wherein the concentration minimum is below125% of the saturation concentration and above 75% of the saturationconcentration.
 6. The method according to claim 1, wherein theconcentration minimum is below 100% of the saturation concentration andabove 50% of the saturation concentration.
 7. The method according toclaim 1, wherein substantially no aluminum hydroxide precipitate isformed by homogeneous precipitation.
 8. The method according to claim 1,wherein the electrolyte includes a hydroxide base at a concentration ofbetween about 0.1 M to 3 M.
 9. The method according to claim 1, whereinthe electrolyte includes a hydroxide base at a concentration of betweenabout 0.25 M to about 2.5 M.
 10. The method according to claim 1,wherein the water includes sodium chloride.
 11. The method according toclaim 1, wherein the aluminum has a purity of at least 99.95 wt %. 12.The method according to claim 1, wherein the aluminum in the alloy has apurity of at least 99.99 wt %.
 13. The method according to claim 1,wherein the aluminum or aluminum alloy is substantially free of tin andboron.
 14. The method according to claim 1, wherein the cathode is inthe form of a solid plate.
 15. The method according to claim 1, whereinthe cathode is in the form of a sponge.
 16. The method according toclaim 1, wherein the cathode includes a nickel-molybdenum alloy.
 17. Themethod according to claim 1, the cathode having a surface characterizedby an electrochemical roughness factor of at least
 5. 18. The methodaccording to claim 1, the cathode having a surface characterized by anelectrochemical roughness factor of at least
 10. 19. The methodaccording to claim 1, substantially free of aerobic oxidation of thealuminum or aluminum alloy.
 20. The method of according to claim 1,further comprising: providing a waste separation system, in fluidcommunication with the housing, configured to receives the electrolyteand aluminum hydroxide waste from the electrochemical cell and toseparates the aluminum hydroxide waste from the electrolyte, andproviding a fuel injector, in fluid communication with the wasteseparation system and the water injection port, configured to receivesthe electrolyte from the waste separation system and to provides theelectrolyte to the water injection port.
 21. The method according toclaim 20, wherein the fuel injector is further configured to receiveswater from a water supply.